Zirconium(IV)-modified silica gel: Preparation, characterization and catalytic activity in the synthesis of some biologically important molecules

Article abstract of DOI:10.1016/j.catcom.2010.10.011

Zirconium modified silica gel was prepared by the grafting method and the resulting organic-inorganic hybrid material was found to be a highly effective catalyst for the range of organic transformations such as syntheses of coumarins, quinoxalines and 2,4

Full text of DOI:10.1016/j.catcom.2010.10.011

Catalysis Communications 12 (2011) 327–331
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Zirconium(IV)-modified silica gel: Preparation, characterization and catalytic activity
in the synthesis of some biologically important molecules
⁎
R.K. Sharma , Chetna Sharma
Department of Chemistry, University of Delhi, Delhi- 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2010
Received in revised form 8 October 2010
Accepted 12 October 2010
Available online 16 October 2010
Zirconium modified silica gel was prepared by the grafting method and the resulting organic–inorganic hybrid
material was found to be a highly effective catalyst for the range of organic transformations such as syntheses
of coumarins, quinoxalines and 2,4,5-trisubstituted imidazoles. The low percentage of zirconium in the
catalyst gave products in high turnover numbers; the catalyst was recovered after reaction, and reused. The
structure of the catalyst was investigated and confirmed by surface area (BET), FT-IR, ED-XRF and elemental
analyses, 13C-CPMAS spectral studies, molecular mechanics studies and inductively coupled plasma mass
spectrometry technique (ICP-MS).
Keywords:
Organic–inorganic hybrid catalyst
Grafted
© 2010 Elsevier B.V. All rights reserved.
Zirconium
Coumarins
Quinoxalines
Imidazoles
1. Introduction
alines, and one-pot multi-component synthesis of 2,4,5-trisubstituted
imidazoles. The biological, pharmaceutical, medicinal, and therapeutic
The catalysts which would be useful to make the organic
transformations eco-friendly and economically viable are highly
demanded by academic laboratories and industries [1]. In this respect,
development of immobilized catalysts that allows the simple work-up
procedure and easy separation from reaction products is challenging
and important [2]. In fact, the last decade has witnessed a growing
interest in the building up of organic–inorganic hybrid catalysts using
several types of supports and several immobilization strategies [3–8].
The importance of zirconium as a homogeneous catalyst has already
been cited in the literature in the number of biologically and
pharmaceutically significant organic transformations [9–14]. Our
present work is directed to immobilize, for the first time, zirconium
complex onto the functionalized support thereby combining the
properties such as catalyst selectivity and activity with the ease of
separation and catalyst reuse. As the choice of support material plays a
critical factor in the performance of the resulting supported reagent
catalyst, we have chosen silica gel support due to its high surface area,
excellent stability (chemical and thermal), good accessibility, and
ease of functionalization of the surface groups [15,16].
properties of coumarins [17], quinoxaline derivatives [18], and substituted
imidazoles [19] have been well established in the literature. Although
there are many synthetic routes to obtain these derivatives, they suffer
from several limitations such as drastic reaction conditions, low product
yield, tedious work-up procedures, the use of toxic metal salts as catalysts,
and expensive reagents. The main disadvantage of almost all existing
methods is that the catalysts are destroyed in the work-up procedure and
cannot be recovered or reused [20,21].
2. Experimental
2.1. Materials
Zirconium oxychloride octahydrate (Aldrich), 3-aminopropyl-
triethoxysilane (Fluka), silica gel (Qualigens), and 5-chloro salicylal-
dehyde (Aldrich) were commercially obtained and used as such in
this study. Starting materials and reagents used in the reactions were
obtained commercially from Alfa Aesar and Spectrochem Pvt. Ltd. and
used without purification.
In the present study, Zr-CAP-SG has been investigated for its catalytic
activity in the number of significant organic transformations such as
Pechmann condensation of phenols with β-ketoesters to yield coumarins,
condensation of various 1,2-diamines with 1,2-diketone to give quinox-
2.2. Characterization techniques
The IR spectra were recorded on Perkin Elmer Spectrum 2000
Fourier transform infrared (FT-IR) spectrometer. Surface area analysis
was carried out at 77 K by Model 2010, Micromeritics, USA. Elemental
analysis was performed on an Elementar Analysensysteme GmbH
VarioEL V3.00. 13C-CPMAS spectra were recorded on a Bruker DSX-300
⁎
Corresponding author. Tel./fax: +91 11 27666250.
E-mail address: rksharmagreenchem@hotmail.com (R.K. Sharma).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.10.011

328
R.K. Sharma, C. Sharma / Catalysis Communications 12 (2011) 327–331
Table 1
Pechmann condensation of various phenols with β-keto ester using Zr-CAP-SG as the catalyst.a
O
OH
O
O
O
C
cat, neat
130 o
C
C
H3C
OEt
R1
R1
Entry
R1
Time/min
Yieldb/%
1.
2.c
3.
4.
5.
6.
3-OH; 5-OH
3-OH; 5-OH
2-OH; 3-OH
3-OH
3-Me
4-NO2
5
5
5
30
30
480
N99
98, 96, 96, 94, 93
N99
94
96
0
a
Phenol (1 mmol), β-keto ester (1 mmol) and 10 wt.% catalyst were stirred at 130 °C.
Isolated yields.
Recycling experiment.
b
c
NMR spectrometer at 75.47 MHz. The content of zirconium in the
heterogenized catalyst was determined by an ICP-MS Agilent 75003
model number G3272A. Molecular Mechanics calculations were
performed using Spartan'08, Wavefunction, Inc. [22]. Qualitative
analysis of catalyst for heavy metals was performed by XRF using
energy dispersive XRF spectrometer (Fischerscope X-ray XAN-FAD BC).
1H NMR spectra for the products were recorded on Bruker 300 MHz
instrument using TMS as internal standard. Melting points were
recorded on a Buchi R-535 apparatus and are uncorrected.
important organic transformations such as C–O bond formation reaction
(Pechmann condensation for the synthesis of coumarin derivatives)
(Table 1), C–N bond formation reaction (synthesis of quinoxaline
derivatives) (Table 2) and multi-component synthesis of trisubstituted
imidazoles (Table 3).
2.3.1. Preparation of [1,2-bis(5-chlorosalicylidene amino)-phenylene],
CAP, 1
The complexing agent, 1 was prepared by the condensation reaction
of o-phenylenediamine (0.54 g, 5 mmol) with 5-chloro salicylaldehyde
(1.56, 10 mmol) in ethanol (10 mL) for 3 h. After the completion of the
reaction, a yellow precipitate of the product obtained was filtered and
dried (86%) and recrystallized from acetonitrile. 1H NMR (d6-acetone,
2.3. Preparation of catalyst and its applications
The grafted catalyst was prepared by following the slightly modified
method of Airoldi et al. [23] and was investigated for a number of
300 MHz): δ (ppm) 8.92 (s, 1H), 8.91 (s, 1H), 6.98–7.67 (m, Ar–H). 13
C
Table 2
Condensation of various 1,2-diamines with 1,2-diketone in ethanol at room temperature using Zr-CAP-SG as the catalyst.a
O
O
N
N
cat, RT
R
a
NH2
NH2
a
EtOH
or
+
or
O
O
R
N
N
R
b
b
Entry
R
Product
Time/h
Yieldb/%
1.
2.c
3.
4.
5.
6.
7.
8.
H
H
1a
2a
3a
4a
5a
6a
7b
8b
1.5
1.5
1.0
4.5
3.0
2.5
1.7
2.0
92
90, 90, 89, 90, 88
4-Me
Ethylene diamine
4-PhCO
4-Cl
H
95
86
83
85
89
87
4-Me
a
1,2-diamine (1 mmol), benzil (1 mmol) and 10 wt.% catalyst were stirred in 2 mL of ethanol at room temperature.
Isolated yields.
Recycling experiment.
b
c

R.K. Sharma, C. Sharma / Catalysis Communications 12 (2011) 327–331
329
Table 3
3.77; and N, 7.19. Found: C, 62.35; H, 3.66; and N, 7.27. IR (KBr):
max =3445, 3366, and 1615 cm−1
One-pot condensation of various aldehydes with 1,2-diketone and ammonium acetate
ν
.
in acetonitrile at room temperature using Zr-CAP-SG as the catalyst.a
R1
O
O
R1
2.3.2. Grafting procedure for silica, 2
N
cat, RT
Silica gel was functionalized using 3-aminopropyltriethoxy silane to
yield aminopropyl silica gel (APSG) according to a reported procedure
[16]. 13C CPMAS NMR analysis: 9.3 (–Si–CH2–), 22.3 (–CH2–), and 42.9
(–N–CH2–) ppm. IR (KBr): νmax=3434, 2924, 2851, 1630, 1083, and
798 cm−1. A mixture of APSG (4 g) and ligand (1.54 g, 8 mmol), 1 was
refluxed in a diglyme solution for 3 h at 140 °C under nitrogen [23].
It was then filtered off and washed thoroughly with acetonitrile
and acetone. The material obtained was dried overnight at 90 °C. 13C
CPMAS NMR analysis: 9.2 (–Si–CH2–), 22.7 (–CH2–), 41.1 (–N–CH2–),
50.1 (–N–CH2–), 59.1 (–N–CH2–), 118.9 (Ar–C), 122.5 (Ar–C), 130.2
(Ar–C), 159.8 (C–O), and 163.1 (C=N) ppm. IR (KBr): νmax=3422,
R2CHO
NH4OAc
CH3CN
R2
R1
R1
N
H
Entry
R1
R2
Time / h
Yieldb/%
1.
2.c
3.
4.
5.
6.
7.
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4.45
4.45
8.45
6.0
7.0
6.5
87
C6H5
4-Me
4-Cl
4-NO2
4-Br
87, 85, 86, 84, 84
71
79
73
78
86
5.5
2927, 1641, 1096, and 801 cm−1
.
O
2.3.3. Preparation of catalyst, 3
8.
C6H5
5.0
89
The grafted silica gel, 2 (4 g) was stirred with zirconium(IV)
oxychloride (0.258 g, 0.8 mmol) in acetonitrile for 2 h at 80 °C (Scheme 1).
It was then filtered off and washed thoroughly with acetone until the
washings were colourless. The catalyst thus obtained was dried in a
vacuum oven overnight at 90 °C. Anal. Found: C: 9.53, H: 2.15, and N: 1.58.
IR (KBr): νmax =3431 (–OH str.), 2922 (–CH2– str.), 1637 (–C=N– str.),
N
a
Aldehyde (1 mmol), benzil (1 mmol), ammonium acetate (2 mmol) and 10 wt.%
catalyst were stirred in 2 mL of acetonitrile at room temperature.
b
Isolated yields.
Recycling experiment.
c
and 1092 (–Si–O–Si– str.) cm−1. Surface area analysis: 141.93 m2 g−1
ICP-MS analysis: amount of zirconium: 0.17 mmol g−1
.
.
3. Results and discussion
3.1. Catalyst characterizations
NMR (d6-acetone, 300 MHz): δ (ppm) 163.8 (C=N), 159.9 (C–O), 118.6
(Ar–C), 118.7 (Ar–C), 119.9 (Ar–C), 120.4 (Ar–C), 128. 3 (Ar–C), 131.7
(Ar–C), 132.9 (Ar–C). Elemental analysis: C20H14Cl2N2O2: C, 62.35; H,
The three signals at 9.3, 22.3 and 42.9 ppm observed in the 13C-
CPMAS NMR spectrum of APSG due to –Si–CH2–, –CH2– and –N–CH2–
HO
OH
HO
NH2
ethanol
2
Cl
Cl
3h, room temp.
N
C
H
C
H
N
NH2
OHC
Cl
1
APSG
3h 140 oC
diglyme, N2
OH
H
HO
H
N
N
ZrOCl2.8H2O
CH3CN
C N
N C
H
H
O
O
Si
O
O
Si
2h, 80o C
O
O
2
O
O
O
Zr
H
N
H
N
C N
N C
H
H
O
O
O
Si
O
O
O
Si
3
Scheme 1. Synthesis of covalently anchored zirconium complex onto silica.

330
R.K. Sharma, C. Sharma / Catalysis Communications 12 (2011) 327–331
respect to the support cannot be ruled out. As a supporting evidence,
energy minimization was carried out using MMFF force field (Fig. 2).
The energy minimized structure of a monomeric unit supports the
observation that the molecule is not symmetrical with respect to the
support. In addition, other signals in the spectrum can be assigned to –
C=N– (163.1 ppm), and –C–O– (159.8 ppm) and the peaks at 118.9 ppm,
122.5 ppm, and 130.2 refer to the aromatic carbons of the phenyl rings.
No significant changes are observed in the silica gel structure
sensitive vibrations, after its modification with 3-aminopropyltriethoxy
silane, which is an indication that its framework remained unchanged.
The spectrum of APSG exhibits an additional band at about 2924 cm−1
and 2851 cm−1 due to the aliphatic (–CH2) stretching of the propyl
chain of the silylating agent, thus suggesting that the silica gel surface
was functionalized with the linking agent. The infrared spectrum of the
1,2-bis(5-chlorosalicylidene amino)-phenylene exhibits two sharp
bands at 3445 cm−1 and 3366 cm−1 attributed to the presence of two
free hydroxyl groups. The band at 1615 cm−1 is due to the C_N
stretching vibration of the imine bond. The spectrum of chemically
modified silica shows a large number of bands which merely appeared
as shoulders due to its superimposition by a broad polymer band at
1092 cm−1. The bands at 2922 cm−1 and 1637 cm−1 are assigned to
aliphatic –CH2 and C_N stretchings.
Fig. 1. 13C NMR spectrum of CAP-SG.
The decrease in the surface area of the catalyst [24] is found to be
40% which is indicative of the grafting of ligand and hence zirconium
oxychloride onto the silica gel. Chemical analysis of APSG (Anal.
Found: C: 5.28, H: 1.26, and N: 2.04) corresponds to 1.46 mmol/g of 3-
aminopropyl groups based on the nitrogen percentage, whereas for
the catalyst the loading of organic moiety onto the silica gel is only
13.2% enabling a theoretical capacity of 0.18 mmol g−1 of Zr-CAP-SG.
The metal loading of Zr-CAP-SG, was confirmed by ICP-MS and found
to be 0.17 mmol/g. Further to support the above observation, the
catalyst was subjected to energy dispersive X-rays (Fig. 3).
3.2. Catalytic studies
Fig. 2. A perspective view of the monomeric unit of CAP-SG.
To investigate the catalytic activity of the organic–inorganic hybrid
material different reactions were studied. The reactions did not proceed
in the absence of the catalyst or only silica as the catalyst. However, Zr-
CAP-SG served as an efficient catalyst for the reactions with high turnover
numbers. As can be seen (see Supplementary data), ethanol clearly
stands out as the solvent of choice for the synthesis of quinoxalines
derivatives with its high reaction rate, high turnover number, selectivity,
low cost, and environmental acceptability. However, acetonitrile proved
groups, respectively, authenticate the synthesis of APSG. Moreover,
the covalent binding of ligand 1 with APSG has been confirmed by the
shifting of –NH–CH2– peak to 50.1 ppm and 59.1 (Fig. 1). The two
values confirm that grafting occurred at two different points of
attachment. Since the 13C NMR chemical shifts are far more sensitive
to changes in electronic and steric effects, the possibility of these two
different values due to different orientation of phenyl ring with
Fig. 3. XRF-spectrum of the catalyst (showing resolved peak of zirconium).

R.K. Sharma, C. Sharma / Catalysis Communications 12 (2011) 327–331
331
to be a better solvent for the preparation of substituted imidazoles since
the reaction time period is least in this case. The Pechmann condensation
of coumarins showed a drastic change in the reaction time period and
product yield with increasing temperature. Therefore, we tried this
particular reaction under neat conditions and at higher temperatures. For
the other two reactions, there were not much significant changes
observed in product yields or reaction time period with higher
temperatures. As far as catalyst recovery is concerned, for every cycle it
has been recovered, regenerated and recycled. There has been no loss in
the amount of the catalyst after its recovery. It is worth mentioning here
that for every cycle the catalyst has been tested for the reaction at
microscale.
In order to verify whether the observed catalysis is truly
heterogeneous or not, the catalytic reactions were carried out using
Zr-CAP-SG as the catalyst under the conditions indicated in Tables 1, 2,
and 3 and the solid Zr-CAP-SG was removed by filtration at about half
the specified reaction time period. Then, the filtrate was allowed to
react under the same conditions. No increase in the yield was
observed. This suggests that the nature of the observed catalysis is
truly heterogeneous.
Institute of Science (IISc), Bangalore for performing 13C NMR
measurements.
Appendix A. Supplementary materials
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2010.10.011.
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Acknowledgements
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One of the authors, C. Sharma thanks the Council of Scientific and
Industrial Research (CSIR), New Delhi, India, for the award of Junior
and Senior research fellowships. The authors also thank the Indian
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